The mineralized tissue of the bones of the human skeletal system are generally categorized into two morphological groups: “cortical” bone and “cancellous” bone. The outer walls of all bones are composed of cortical or “compact” bone. This tissue is characterized by a dense structure with only microscopic porosity. Cancellous or “trabecular” bone is found in the interior of bones. This tissue is composed of a lattice of interconnected slender rods and plates called “trabeculac.”
Injectable Polymethylmethacrylate (PMMA), set within the trabeculae, has been used for supplementing cancellous bone, especially for anterior and posterior stabilization of the spine in metastatic disease. See Sundaresan, et al., “Treatment of neoplastic epidural cord compression by vertebral body resection and stabilization,” J Neurosurg 1985;63:676-684; Harrington, “Anterior decompression and stabilization of the spine as a treatment for vertebral collapse and spinal cord compression from metastatic malignancy.” Clinical Orthodpaedics and Related Research 1988;233:177-197; and Cybulski, “Methods of surgical stabilization for metastatic disease of the spine.” Neurosurgery 1989;25:240-252. Deramond et al., “Percutaneous vertebroplasty with methyl-methacrylate: technique, method, results [abstract],” Radiology 1990; 117 (suppl): 352, among others, have described the percutaneous injection of PMMA into vertebral compression fractures by the transpedicular or paravertebral approach under CT and/or fluoroscopic guidance.
Percutaneous vertebroplasty is desirable from the standpoint that it is minimally invasive as compared to the alternative of surgically exposing a hard tissue site to be supplemented with PMMA or other filler. Several procedures are known for accessing a desired site in the cancellous bone of a vertebral body (or for that matter other cancellous bone) to deliver hard tissue implant material to stabilize—or build up—a site once expanded as taught by U.S. Pat. Nos. 6,280,456; 6,248,110; 5,108,404 and 4,969,888.
To gain access to a hard tissue implantation site, as described in U.S. Pat. No. 6,019,776 and 6,033,411, a straight needle or cannula in combination with a stylet may be employed. Once access is achieved and the stylet is removed from the cannula, hard tissue implant material is delivered through the same.
Another approach for biopsy sampling or material infusion is employed with a product sold by Cook Medical, Inc. The approach involves the use of a straight cannula/stylet combination for gaining access to the cancellous bone and a curved Nitinol (NiTi) needle for accessing a site that is radially oriented from the end of the cannula. The full set of instruments sold by Cook under the OSTEO-RX™ produce line is shown in FIG. 1. It includes straight cannula 2, a stylet 4 and obturator 6 for receipt in the cannula—each made of stainless steel. It further includes a curved Nitinol needle 8 and a flexible stylet 10 for receipt in the curved needle. The cannula/introducer needle 2 is a 10-gage member, 10 cm in length; the curved needle is a 13-gage member, 19 cm in length.
In use, cancellous bone tissue is accessed by traversing compact bone tissue with the stylet/cannula combination 12, each having a beveled end as highlighted in the magnified end images. Once a desired depth within the bone is reached, the stylet is withdrawn. The obturator 6 may be placed in cannula 2 in order to close-off the cavity temporarily or clear out tissue invading its space upon withdrawal of stylet 4. At this point, curved needle 8 is introduced into cannula 2.
Made of superelastic material, the needle straightens as the walls of the cannula apply force against it. Whether stylet 10 is inserted before or after loading the cannula with the curved (now straightened) needle 8, the combination is advanced in cannula 2 so that the distal end of the stylet, needle combination 14 bores through cancellous bone tissue a the desired delivery site. At this point, stylet 10 is removed to allow infusion of implant material under pressure to the site. Alternately, stylet 10 is only partially withdrawn and needle 8 advanced further to take a biopsy sample.
Whatever the intent of the procedure, needle 8 is then withdrawn at least into the body of the cannula so the straight cannula section may then be removed from the patient's body. For certain reasons, it may be desirable to remove the needle from the cannula altogether, e.g. for later infusion of material through the canula when the needle is used for acquiring a biopsy sample. In either case, no conduit is left behind to allow infusion of an implant material specifically at the end of the curved needle's tract.
Especially when withdrawing the curved needle from the cannula (but also in loading it into the cannula), the system presents high risk to users. It has been observed that the stiffness of curved needle 8 and amount of energy it stores upon straightening mandates extreme caution in handling, lest injury result from its curved end 16 returning to its unconstrained, curved shape—impaling the user. In addition to the user safety issues the device presents, the difficulty presented in straightening needle 8 within cannula 4 produces significant frictional forces between the members resulting in less than optimal actuation and control of the system. Furthermore, the insertion or removal (at least partial removal) activity of the curved needle into and out of the cannula occurs when cannula 2 is set within a patient's body makes any such manipulation more difficult.
Except for the present invention, no known solution has been developed that provides functionality like the above-referenced system, but without the noted problems with safety and ease use. As such, the present invention is particularly suited to meet the needs of bone access at sites that are radially located from an access path through harder bone tissue. It does so through operation principles which differ from those of the Cook system. Accordingly, a conduit and core member used in the present invention each differ in their unstressed shape and material properties as compared to those in the other system.